Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include transmitters having a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Receivers are also provided to demultiplex the received WDM signal into individual optical signals, convert the optical signals into electrical signals, and output the data carried by those electrical signals.
Conventionally, WDM systems have been constructed from discrete components. For example, demultiplexer and photodiodes have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components, have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated optical output.
One such optical signal modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide spectral densities with higher data rates per unit of fiber bandwidth than other modulation formats, such as on-off keying (OOK).
A receiver configured to decode and output the information carried by a Pol Muxed DQPSK signal is described in U.S. patent application Ser. Nos. 12/052,541; 12/345,817; and 12/345,824, the entire contents of each of which are incorporated herein by reference. In such systems, various components are provided on multiple substrates. There is a need, however, to provide such components of an optical receiver, which receives Pol Muxed DQPSK modulated optical signals or optical signals modulated in accordance with other modulation formats, on one substrate to improve reliability, simplify manufacturing, and reduce costs.
In addition, a polarization beam splitter may be provided that receives optical signals having first and second polarizations at an input port, and outputs the optical signals having one polarization, e.g., a TE polarization, on a first output, and those optical signals having the other polarization, e.g., TM polarization, are output separately on a second output port. In a DQPSK system, for example, the optical signals may be supplied to a known optical hybrid circuit, which, as generally understood, outputs components, such as the quadrature (Q) and in-phase (I) of the input optical signals. As is further generally understood, the polarization states of the incoming optical signals supplied to the optical hybrid circuit are preferably the same, in order to adequately recover the data carried by the optical signals. Thus, one of the TE and TM signals output from the polarization beam splitter, e.g., the TM signal, is supplied to a rotator so that the TM polarization is rotated to a TE polarization state. As a result, both signals supplied to the optical hybrid circuit nominally have the same TE polarization.
Often, however, undesired residual or extraneous TM light may be included with the TE light output from one port of the polarization beam splitter, and similarly, a relatively small amount of TM light may be output from the rotator. Such residual TM light may result in errors in data recovered from the optical signals. Accordingly, there is a need to reduce or suppress such residual TE and TM light.